Ktao3 ultraviolet detector

ABSTRACT

SOLID STATE PHOTO-CONDUCTIVE AND PHOTO-VOLTAIC ULTRA VIOLET DETECTORS HAVING A PEAK RESPONSE AT ABOUT 2600 TO 2800 A. ARE FORMED A BODY OF POTASSIUM TANTALATE (KTAO3) HAVING AN &#34;AS-GROWN&#34; SURFACE AND FIRST AND SECOND METAL CONTRACTS ATTACHED TO THE AS-GROWN SURFACE, FORMING SURFACE BARRIER JUNCTIONS. THE RADIATION WHICH IS DETECTED IS INCIDENT UPON THE AS-GROWN SURFACE.

Feb. 20, 1973 w, P N 3,717,799

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Feb. 20, 1973 P. W. CHAPMAN KIAO ULTRAVIOLET DETECTOR Filed Feb. L9,1971 7 Sheets-Sheet 7 SILVE PAINI: CONTACT |LVER SILVER DA|NT PAINTCONTACT KTao CONTACT REVERSE BIASED JUNCTION FORWARD BIASED JUNCTIONFIG. ICB

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A 7' TOR/VEX United States Patent Ofice 3,717,799 Patented Feb. 20, 19733,717,799 KTaO ULTRAVIOLET DETECTOR Paul W. Chapman, Burnsville, Minn.,assignor to Honeywell Inc., Minneapolis, Minn. Filed Feb. 19, 1971, Ser.No. 116,844 Int. Cl. H01l15/00, 15/02 US. Cl. 317-234 R 13 ClaimsABSTRACT OF THE DISCLOSURE Solid state photo-conductive andphoto-voltaic ultraviolet detectors having a peak response at about 2600to 2800 A. are formed from a body of potassium tantalate (KTaO having anas-grown surface and first and second metal contacts attached to theas-grown surface, forming surface barrier junctions. The radiation whichis detected is incident upon the as-grown surface.

BACKGROUND OF THE INVENTION Ultraviolet detectors have found applicationin systems which monitor ultraviolet radiation levels for eye safety,ultraviolet radiometers for terrestial and space application and systemswhich utilize ultraviolet absorption of certain vapors as a means forvapor detection. However, one of the most important applications forultraviolet detectors at the present time is in the general area offlame detection.

The most reliable and universal method of monitoring the presence orabsence of a flame is to detect the ultraviolet radiation emitted by theflame. A first advantage of an optical detection method for flamesensing over the more conventional methods using thermal sensors such asthermocouples or thermistors is that optical sensors have response timesof a few milli-seconds or less, Where thermal detectors have responsetimes of the order of seconds. Second, an optical sensor can be placedat a remote location where it is not subject to high temperatures orcorrosion. Third, an optical sensor can detect flames anywhere withinthe field of view of the detector and over large distances, so that itis not necessary to know the exact position of the anticipated flame.This feature is important in fire detection and tracking systems.Ultraviolet detection for flame sensing is preferable over detection inthe infrared portion of the spectrum because an infrared detector cannotdistinguish between a flame and a hot body. Detection in the visibleportion is undesirable because both sunlight and artificial ambientlight cause the detector to respond. In addition, the intensity of blackbody radiation falls off exponentially from the visible to theultraviolet range. In contrast to infrared and visible radiation, theultraviolet emission of a flame gives a unique signature which can berapidly detected by a suitable sensor.

Referring to FIG. 1, the emission spectra of several different flames isshown. While the exact spectrum for each flame is dependent upon theexact conditions present, such as fuel to air mixture, it can be seenthat strong emission peaks for each of the flames occurs atapproximately 3100 A. and the relative intensity of each decreasesrapidly with decreasing wavelength for wavelengths below 2800 A.

The ultraviolet radiation observed is generated in the reaction zone ofthe flame and is a result of electronic molecular transitions and is notof a thermal character. Therefore its spectral intensity can be muchhigher than the spectral intensity corresponding to the thermalradiation of a black body heated to flame temperature. The ultravioletradiation is associated with the rearrangement of electronic molecularorbitals during reaction. It thus derives its energy directly fromcertain chemical reactions and is therefore termed chemiluminescent. Thedominant emissionshown by the flames in FIG. 1 at 3060 A. and at 2810 A,is due to the excited states of the OH molecule. Flame spectra becomemore complex as more atomic species such'as the halogens and metals areadded to the reaction. However, the mechanisms for generating molecularultraviolet radiation remain very similar.

A principal problem associated with the use of the ultraviolet emissionas a means for flame detection is that the flame emits a relativelysmall amount of ultraviolet radiation, consequently very sensitiveultraviolet detectors are required for many applications. In addition,the magnitude of the energy emitted by the flame in the ultravioletportion of the spectra is often much smaller than the energy present inthe visible and infrared portions of the spectrum due to artificiallights and solar radiation; therefore it is necessary for theultraviolet detector to be sensitive to ultraviolet radiation andinsensitive to any ambient radiation that is present. In many cases, thedetector is used in direct sunlight, and therefore the long wavelengthcutoff must be below approximately 2850 A. In all applications it isdesirable to have a detector response curve which has a sharp longwavelength cutoif and a high degree of sensitivity immediately below thecutoff wavelength, since the relative emission intensity of the flamesdecreases rapidly with decreasing wavelength.

Two commonly used ultraviolet radiation detectors are photo multipliertubes and cold cathode gas-filled tubes. While the photo multipliertubes are capable of extremely high sensitivity, they have thedisadvantages of high cost, high voltage requirements, ease of breakageand vibrational damage, and the lack of a sharp long wavelength cutoifwhich results in sensitivity in the visible portion of the spectrum.

The cold cathode ultraviolet tubes are gas discharge tubes which ignitewhen irradiated with ultraviolet radiation. These tubes, which arepresently used in furnace systems, have a high degree of sensitivity andwith correct choice of cathode material, have a significant responseonly below 2800 A. The discharge is initiated by the photoelectriceffect at the cathode of the tube, and consequently the spectralsensitivity increases slowly with decreasing wavelength. This lack of along wavelength cutoff requires that the peak sensitivity of the tube bewell below the cut-off wavelength imposed by the ambient light. The coldcathode ultraviolet tubes have the further disadvantages of ease ofbreakage and vibrational damage, high voltage requirements, anddifliculty in obtaining reliable tubes with a response beyond 3000 A.due to damage of the cathode by ion bombardment.

Prior Art Solid State Ultraviolet Detectors Solid state ultravioletdetectors offer a number of potential advantages including ruggedness,compatibility with integrated circuitry, small size and long life.

Despite its potential advantages, a solid state detector having responseto ultraviolet radiation only and capable of sensing extremely lowintensity radiation in the DC mode as well as the AC mode of operationhas not heretofore been available.

In the DC mode of operation the detector responds to uninterruptedradiation from a steady state source of radiation. The signal from thedetector is amplified by standard DC methods well-known in the art. Thebasic restriction on a detector operating in the DC mode of operation isthat the dark current, in other words that current flowing through thedetector when no radiation is incident upon it, must be much smallerthan the photocurrent produced when radiation is incident. In the priorart it has been possible to fabricate solid state detectors which arecapable of responding only to ultraviolet radiation and which are highlyresistive (about to 10 ohms) and therefore have very low dark current,However, these prior art solid state detectors have not had suflicientsensitivity to produce, in response to radiation from extremely lowintensity sources such as gas pilot flames, photocurrents which arelarge enough to be detected when the detector is operated in the DCmode.

In the AC mode of operation, the incident radiation is periodicallyinterrupted and the resultant periodic signal from the detector isamplified by well-known AC methods. The AC mode of operation providesincreased sensitivity to extremely low intensity radiation. However, theAC mode of operation has the disadvantages of increased cost andcomplexity of the radiation detection system.

The fabrication of a suitable solid state ultraviolet detector isfurther complicated by other properties, in addition to low sensitivitywhich characterize large energy gap materials. First, the large energygap materials are characterized by extremely fast free hole-electronrecombination rates, which precludes in most cases any useable bulkphoto-conductive effects. Second, most large energy gap materials arenon-amphoteric; in other words they exist normally as only a singleconductivity type, either N type or P type. Therefore, with thesematerials it is extremely difficult to fabricate PN junctions suitablefor radiation detection.

Solid state ultraviolet detectors using silicon carbide (SiC) can bemade both N and P type and consequently SiC detectors are normally madeas photodiodes. However, the energy gap of SiC is only 3 electron volts,and therefore these detectors always have some sensitivity atwavelengths out to 4100 A. Therefore SiC is an unsuitable detectormaterial for many applications, since it is sensitive to both thevisible and ultraviolet portions of the spectrum. In order to isolatethe ultraviolet response by the use of filters, eflicient filters whichtransmit high energy photons and absorb low energy photons are required.Also, in many applications the amount of ambient radiation in thevisible portion of the spectrum is several orders of magnitude largethan the amount of ultraviolet radiation and therefore the filters mustshow a very large rejection for visible light. For these reasons,suitable filters for isolating the ultraviolet portions of the spectrumare extremely diflicult to make.

Another solid state material which is used as an ultraviolet detector iszinc sulphide (ZnS), which has an energy gap of 3.6 electron volts, sothat the cutoff wavelength of undoped material is 3400 A. However, ZnSmust be activated with donor and acceptor impurities to make itphotosensitive. These impurities shift the peak response of the detectorto 3700 A., thereby making the cell sensitivity extend beyond 4000 A. Asin the case of SiC, ZnS is not a suitable ultraviolet detector materialfor many applications.

Potassium tantalate, (KTaO is a wide band gap (approximately 3.5electron volts) extrinsic N type semiconductor with the cubic perovskitestructure. The materials of the perovskite family include KTaO lithiumniobate (LiNbO strontium titanate (SrTiO and several other compoundshaving the general formula ABO The energy gap of these materials rangesfrom 2.8 to 3.7 electron volts. The photoresponse of KTaO wasinvestigated by Wemple, Kahng and Braun in the Journal of AppliedPhysics, January, 1967, pages 353-359. FIG. 2 shows the system used tomeasure the photoresponse of the metal-semiconductor junctions formed bygold, platinum, palladium, indium, nickel and copper dots which werevacuum deposited on a freshly cleaved [100] surface of KTaO An ohmicback contact of chromium covered by a protective overlay of gold wasformed on another cleaved or etched surface. As can be seen in FIG. 2,the radiation was incident upon the backside,

that surface which was opposite the metal-semiconductor junction.

FIG. 3 shows the relative photoresponse as a function of wavelength fora metal-semiconductor junction on KTaO which is illuminated from thebackside as shown in FIG. 2. The peak Wavelength response occurs at 3500angstroms. This is due to the fact that the light incident upon thedetector must pass through the bulk KTaO before reaching themetal-semiconductor junction and therefore only that part of the lightWhose energy is less than the energy gap reaches the metal-semiconductorjunction. It is believed that this response is due to the excitation ofelectrons in the metal over the surface barrier. Since the barrierpotential is less than the energy gap of KTaO photoresponse to radiationin the visible portion of the spectrum is possible and the sharp longwavelength cutoff no longer exists. The response shown in FIG. 3substantiates that the energy gap of KTaO corresponds to a peak responseof about 3400 angstroms, since only the light whose energy is less thanthe energy gap reaches the junction.

SUMMARY OF THE INVENTION The present invention provides photo-conductiveand photo-voltaic ultraviolet radiation detectors which are capable ofsensing extremely low intensity radiation in the DC mode as well as theAC mode of operation. The peak response exhibited by the detectors ofthe present invention occurs at about 2600 to 2800 angstroms. Thedetectors comprise a body of highly resistive KTaO having an as-grownsurface upon which two metal C011. tacts are attached. The as-grownsurface is defined as the smooth outside surface of a crystal which isnot modified in any way by mechanical or chemical polishing.

In the case of the photo-conductive detector, which is defined as aphotodetector operating with an external bias voltage applied, the firstand second contacts form essentially identical surface barriermetal-semiconductor junctions. The photo-voltaic detector, whichoperates with no external bias voltage applied, has two differentsurface barrier junctions, and in its preferred form, one of the metalcontacts forms an ohmic contact with the as-grown surface.

When the as-grown surface of either the photo-conductive or thephoto-voltaic detector is illuminated from the frontside, the detectorexhibits an unexpected peak response at about 2600 to 2800 angstroms.Since the energy gap of KTaO is approximately 3.5 electron volts, theexpected peak response of a detector having frontsideillumination isabout 3400 angstroms, which corresponds to that energy gap.

The photo-conductive detector further differs from prior art devices inthe unexpected result that the photoconductive gain, which is defined asthe ratio of the number of excess charges passing between the electrodesto the number of photons absorbed in a given time interval, is greaterthan unity. The Schottky theory for surface barrier devices predictsthat the maximum possible photoconductive gain is unity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows relative emission as afunction of wavelength for several types of flames.

FIG. 2 shows a system for measuring the photo-response of ametal-semiconductor junction on KTaO by illumination from the backside.

FIG. 3 shows the relative photoresponse as a function of wavelength of ametal-semiconductor junction on KTaO which is illuminated from thebackside.

FIG. 4 shows KTaO; photo-conductive ultraviolet detectors of the presentinvention.

FIG. 5 shows the photocurrent as a function of wavelength of the KTaOphoto-conductive detector of the present invention for difierent biasvoltages.

FIG. 6 shows the photocurrent as a function of Wavelength of KTaOphoto-conductive detectors using various contact metals.

FIG. 7 schematically shows the energy levels of KTaO and a metal such assilver before contact, at the instant contact is made, and afterequilibrium has been achieved.

FIG. 8 shows the photo-conductive gain of KTaO photo-conductive detectoras a function of wavelength.

FIG. 9 compares the relative photoresponse as a function of wavelengthof a KTaQ photo-conductive detector illuminated from the backside andfrom the frontside.

FIG. 10 shows a KTaO device used to investigate the role which the bulkKTaO material plays in the photodetection mechanism.

FIG. 11 shows the photocurrent as a function of wavelength of the deviceshown in FIG. 10.

FIG. 12 compares the photocurrent as a function of wavelength of aphoto-conductive device of the present invention using an as-grownsurface of KTaO with the photocurrent of a detector using a cleavedsurface of KTaO a detector using an as-grown surface of KTaO in whichthe surface between the contacts has been eroded, and the device shownin FIG. 10.

FIG. 13 shows a schematic energy diagram of a photoconductive detectorof the present invention with no external bias applied.

FIG. 14 shows a schematic energy diagram of a photoconductive detectorof the present invention with external bias applied.

FIG. 15 shows a schematic energy diagram of a photovoltaic KTaO'detector of the present invention.

FIG. 16 shows the relative photoresponse as a function of wavelength ofa photo-voltaic KTaO detector of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS KTaO photo-conductive detectorFIG. 4 shows a photo-conductive ultraviolet radiation detector in whicha pair of metal contacts 10 and 1 1 are attached to an as-grown surface12 of a highly resistive body of KTaQ 13, thereby forming twometal-semiconductor junctions of the surface barrier type at theinterface of the contact and body. A battery 14 biases the detector andan ammeter 15 measures the current flowing through the detector.Radiation 16 is incident upon the as-grown surface 12.

The metal contacts and 11 are of the same metal and therefore causeessentially identical barrier potentials at the respectivemetal-semiconductor junctions. In order to form barrier potentials, themetal used for the contacts must have a work function which is differentfrom that of KTaO Copper, gold, gallium, silver, indium, chromium,platinum, magnesium and aluminum are examples of metals which can beused to form contacts in the present invention. The contacts can beformed by evaporation or silk screen painting of the metal onto theas-grown surface of the KTaO body. The contacts can be in the form ofclosely spaced dots, or in a specific geometric pattern such as aninterdigited pattern as shown in FIG. 4a. In the preferred embodiment,the contacts comprise evaporated or painted-on dots of silver.

As discussed previously, a solid state detector operatmg in the DC modeof operation must exhibit a photocurrent which is greater than the darkcurrent of the detector. Therefore the present invention requires anundoped or highly resistive body of KTaO in order to have a very smalldark current. Highly resistive KTaO suitable for use in the solid statedetector of the present invention has been grown at an essentiallyconstant temperature of between about 1285 C. and about 1340 C. asmeasured by an uncorrected optical pyrometer. KTaO grown at temperaturesat or near 1285" C. exhibits a greater dark current. KTaO grown attemperatures at or near 1340 C. low dark current but also exhibits agreatly reduced DC photocurrent. It has been found that KTaO crystalsgrown at temperatures between about 1300" C. and about 1320" C. exhibitboth excellent photosensitivity and low dark current, thereby makingsuch crystals the preferred detector material for the present invention.

FIG. 5 shows the photocurrent of the detector shown in FIG. 4 as afunction of wavelength for DC biases of 10, 50, 200 and 600 volts. Themetal contacts 10 and 11 are vacuum deposited gallium contacts. It isbelieved that the sudden turn-on at approximately 3400 angstroms resultsfrom the onset of intrinsic excitation across the energy gap and henceis a measure of the energy gap. The photocurrent, however, continues toincrease as the photon energy is increased to values greater than theenergy gap. If the photocurrent were due solely to the creation ofelectron-hole pairs in the bulk KTaO the photocurrent would not increaseto such a large extent when the photon energy exceeds 3.5 electronvolts, the energy gap of KTaO Instead, the peak response occurs atapproximately 2700 angstroms, a wave-length corresponding to a photonenergy considerably larger than the energy gap of KTaO FIG. 6 shows thephotocurrent as a function of wavelength for KT aO detectors havingcontacts of copper, gold, silver, magnesium and aluminum. All detectorsexhibit a peak response at about 2600 to 2800 angstroms.

In addition to the unexpected peak wavelength described above, it hasbeen found that KTaO photo-conductive detectors similar to that shown inPG. 4 exhibit photo-conductive gain which is greater than unity, whichis the theoretical maximum photo-conductive gain predicted by theSchottky theory for surface barrier devices. The value of this gain issometimes as high as 10 In order to appreciate the significance of thisdiscovery, photo effects in surface barrier devices must be discussed.

When a metal and a semiconductor whose work functions are equal arebrought into contact there is no transfer of charge and the conductionband in the semiconductor remains flat out to the metal-semiconductorinterface. When an electric field is impressed across such a junction,current flows according to Ohms law as J=E so long as the field is lowenough so that this current requirement is lower than the rate at whichelectrons are excited from the metal to the semiconductor due tothermionic emission. Therefore, at sufficiently low electric fields sucha contact behaves ohmically and space charge neutrality existsthroughout the semiconductor. Such a contact is known as an ohmiccontact.

If, on the other hand, the work function of the metal is greater thanthat of an N type semiconductor, electrons flow from the semiconductorto the metal causing an exhaustion or depletion of majority carriers inthe semiconductor near the contact. This forms a blocking contact orsurface barrier potential at the junction and the current through thesemiconductor is controlled by the junction. Except for very lowvoltages the current is nonohmic. FIG. 7 shows the forming of a surfacebarrier junction. In FIG. 7a an N type semiconductor KTaO and a metalsuch as silver are shown before contact. The instant that the silvercomes in contact with the KTaO FIG. 7b, electrons flow from the KTaOinto the metal so as to cause the Fermi energy (Ef) on each side of theinterface to match. As the system reaches equilibrium, FIG. 70, abarrier potential is formed at the interface which prevents further flowof electrons from the KTaO into the metal.

The photoresponse of a surface barrier device comes primarily from twosources. The first occurs when a photon is absorbed in the metal nearthe metal-semiconductor interface and excites an electron over thesurface barrier. This type of response can occur for photon energiesless than the energy gap of the semiconductor and creates at most onephoto-excited carrier per incident photon. Therefore, the maximumphoto-conductive gain possible is unity. The second type ofphotoresponse occurs when a photon is absorbed in the strong fieldwithin the depletion layer in a reverse-biased junction and creates anelectron-hole pair. The hole is swept by the field across the junctioninto the contact, leaving the electron free to flow in the externalcircuit. The barrier potential at the reverse-biased junction blocks theflow of electrons and therefore only one photo-excited carrier perphoton can flow in the circuit. The maximum photo-conductive gain istherefore limited to unity.

Referring to FIG. 8, the photo-conductive gain of a typical KTaOdetector of the present invention is shown as a function of wavelength.For wavelengths corresponding to photon energies less than the energygap, the gain is less than unity. At 3400 angstroms, which correspondsto the energy gap of KTaO the gain is approximately unity, as ispredicted by the Schottky theory of photoresponse for surface barrierdevices. However, the gain increases with decreasing wavelength andpeaks at a value of approximately This indicates that in addition to theprimary photo effects predicted by the Schottky theory, there is presenta secondary photo effect which is not limited to a maximumphoto-conductive gain of unity.

FIG. 9 compares the relative photoresponse as a function of wavelengthof the same KTaO' detector with silver paint contacts which wasilluminated first from the backside and then from the frontside. Inaddition to the different peak wavelengths, it should be noted that thephoto-conductive gain mechanism present in the frontside illuminateddetector is not present when the detector is illuminated from thebackside. Therefore, the magnitude, of the photoresponse of thefrontside illuminated detector is much larger than that of the backsideilluminated detector.

In order to investigate the role which the bulk KTaO material plays inthe photodetection mechanism a thin (approximately 100 angstroms) silvercontact was evaporated on the as-grown surface 21 of a one-millimeterthick KTaO crystal 22 shown in FIG. 10. The opposite surface 23 waslapped and polished and contacted with a thick coat of silver paint 24.A battery 25 biased the detector and an ammeter 26 measured the currentflowing through the detector. The thin silver contact 20 was nearlytransparent to the radiation 27 incident upon the as-grown surface. FIG.11 shows the photocurrent of the detector of FIG. 10 as a function ofwavelength. The response was much lower than the responseof a detectorhaving both contacts on the asgrown-surface and had a sharp peak at anenergy slightly less than the energy gap. Since the metal-semiconductorjunction formed by the transparent contact was the same as the junctionsformed by contacts on an as-grown surface, the difference in thephoto-response was due to a difference in the bulk and surfaceproperties of KTaO The dilference between the as-grown surface and theinterior material of a KTaO crystal was determined by forming two silverpaint contacts side-by-side on an interior surface of a KTaO crystalobtained by cleaving the crystal. ,FIG. 12 shows that the peakphotocurrent of the detector using the cleaved surface was significantlylowerthan that of the detector fabricated with an as-grown surface. Forcomparison the photocurrent as a function of wavelength of the detectorusing the bulk properties of KTaO as shown in FIG. 11 is superimposedupon FIG. 12.

-In order to determine the depth of the layer which causes theunexpectedly large photoresponse, a detector was fabricated on anas-grown surface of KTaO with silver contacts about four millimetersapart. Initially the photocurrent of this detector when illuminated withthe broad band ultraviolet light source and biased at 100 volts was 1X10amps. Using a slurry of number 600 aluminum-oxide grit, a two-millimeterwide section of the as-.

TABLE I Photocurrent with Depth of erosion (mm.): v. bias (amps.)

1x 10- 0.025 3 x10 0.050 1x10 0.175 1X10 0.375 9 x10- The measurement ofthe photocurrent for various depths of erosion indicates that thephotocurrent is greatly enhanced by the presence of an as-grown surfacelayer which is different from the interior material. Due to the largechange in the photocurrent after the initial erosion and relativelysmall change in the photocurrent with further erosion, it appears thatthe depth of the layer is less than 0.025 millimeter. The photocurrentof the detector as a function of wavelength was measured when theerosion depth was 0.175 millimeter and in FIG. 12 this response iscompared to the response of the cleaved and as-grown surface detectors.It should be noted that the photoresponse of the cleaved surface anderoded surface detectors is similar. The detectors are in both casesessentially two metal-semiconductor junctions connected by bulkmaterial. The cleaved detector differs from the eroded detector in thatthe metal-semiconductor junctions are formed on an interior surfacerather than on an as-grown surface. The similarities in responseindicates that either the photocurrent is limited by the resistance ofthe interior surface or that the metal-semiconductor junctions formed onas-grown and interior surfaces are the same.

Although the unique photoresponse mechanism provided by the as-grownsurface is not well understood, one possible phenomenological model forthe photoresponse of the KTaO detector is as follows. Themetal-semiconductor junction acts as a modified surface barrier junctionwhich is modified by the occurrence of hole traps near the junction.These hole traps become filled when ultraviolet light is absorbed by thedetector, thereby forming an attractive potential near the junctionwhich lowers the barrier potential and causes an increase in the currentflow. This barrier potential lowering is a necessary condition forphoto-conductive gains exceeding unity to occur. The as-grown surfaceallows the barrier potential lowering primarily because it allowstrapping states to exist. In addition the as-grown surface appears tohave a higher conductivity than the bulk material thus allowing agreater flow of photocurrent.

KTaO photo-voltaic detector For many applications, and in particular forflame sensing, it is highly desirable that the solid state ultravioletdetector operate in a fail-safe mode. This mode of operation impliesthat the detector is entirely self-biasing, in other wordsphoto-voltaic. The KTaO detectors discussed previously with reference toFIG. 4 have two contacts of the same metal formed on an as-grownsurface. FIG. 13 schematically shows an energy diagram of a KTaOphoto-conductive detector with no external bias voltage applied. Thesolid lines represent the Fermi (E valance band (V.B.) and conductionband (C.B.) energy levels of the detector when no radiation is incident.The dotted lines represent the corresponding energy levels (representedas E VB. and C.B.') when the detector is irradiated with ultravioletlight. If the junctions are perfectly matched, the separation of chargeat each junction is the same. As the charge at each junction builds upit tends to forward bias the junction, causing the barrier potential todiminish. This is shown schematically by the upward movement of theconduction band and valance band. The Fermi energy level must remain ata constant energy difference from the conduction band and therefore italso rises. The extent to which the Fermi energy level shifts under theinfluence of ultraviolet radiation is directly related to the magnitudeof the photovoltage expected from the detector. However, with no biasapplied across the two identical junctions, the barrier potential ateach junction is reduced by the same amount and therefore the overalleffect is zero voltage. This effect is similar to having two voltagesources of the same voltage opposing each other.

When a bias voltage is applied to the photo-conductive detector, theschematic energy diagram is represented by FIG. 14. The forward-biasedjunction presents a negligible barrier potential to the flow ofelectrons compared to the reverse-biased junction. The forward-biasedjunction leakage current swamps out any photocurrents generated at theforward-biased junction. Photons which are absorbed near thereverse-biased junction, however, create hole-electron pairs which aremeasurable since the leakage current across the reverse-biased junctionis very small. As the hole-electron pair is created, the hole is sweptinto the metal contact leaving the electron free to flow in the externalcircuit. That the reverse-biased junction is the only junction active inthe photodetection process is verified by scanning the area between thecontacts with a small beam of light. Photoresponse data obtained by suchscanning indicates that the majority of the photoresponse comes from thearea directly adjacent to the reverse-biased junction.

In view of the operation of the photo-conductive KTaO detector it can beseen that in order to produce a photo-voltaic detector the first andsecond surface barrier potentials must be of different height. Like thephotoconductive detector, a photo-voltaic detector is formed byproviding two contacts on an as-grown surface of KTaO However, thecontacts are formed by different metals such that the first contactforms a first metalsemiconductor junction having a large surface barrierpotential while the second contact of a second metal forms a secondmetal-semiconductor junction having a small surface barrier potential.The second contact must be of low resistance so as not to limit theshort circuit current and response time of the detector. In thepreferred embodiment, the first contact is a painted-on or evaporatedsilver dot, while the second contact is an ohmic contact formed by anultrasonically soldered indium rich alloy. In this embodiment, thesecond surface barrier potential is essentially zero.

FIG. 15 shows a schematic energy diagram for the photo-voltaic KTaOdetector to which no external bias voltage is applied. As in FIG. 13,solid lines represent the energy levels of the detector when noradiation is present and dotted lines represent energy levels of thedetector when ultraviolet radiation is present. When the detector isirradiated with ultraviolet light, the charge separation at the surfacebarrier junction causes a change in the position of the Fermi levelwhich is equal to the open circuit voltage. If a wire were connectedbetween the two contacts, charge would flow through the wire so as toreturn the Fermi level to its original position. This current is theshort circuit current of the detector.

FIG. 16 shows the relative photoresponse as a function of wavelength ofa photo-voltaic detector in which a silver paint contact and anultrasonically soldered 96.5% indium, 3% antimony, 0.5% gallium contactwere formed on an as-grown surface of KTaO Radiation was incident uponthe as-grown surface and no external bias was applied to the detector.As with the photo-conductive detector, the photo-voltaic detectorexhibits a peak response at between 2600 and 2800 angstroms.

While this invention has been particularly shown and 10 described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that changes in form and details may be madetherein without departing from the spirit or scope of the invention.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:

1. A solid state ultraviolet radiation detector comprismg:

a body of highly resistive potassium tantalate grown at an essentiallyconstant temperature of between about 1285 C. and about 1340 C., andhaving an as-grown surface upon which ultraviolet radiation is incident,and

first and second metal contacts attached to the asgrown surface to formfirst and second metal-semiconductor junctions, causing first and secondbarrier potentials at the respective junctions.

2. The solid state ultraviolet radiation detector of claim 1 wherein thebody of highly resistive potassium tantalate is grown at an essentiallyconstant temperature of between about 1300 C. and about 1320 C.

3. The solid state ultraviolet radiation detector of claim 1 wherein thefirst and second metal contacts are formed by the same metal.

4.. The solid state ultraviolet radiation detector of claim 3 whereinthe first and second metal contacts are formed by a metal from the groupconsisting of: copper, gold, gallium, silver, indium, chromium,platinum, magnesium, and aluminum.

5. The solid state ultraviolet radiation detector of claim 4 wherein the[first and second metal contacts are paintedon dots of silver.

6. The solid state ultraviolet radiation detector of claim 4 wherein thefirst and second contacts are evaporated dots of silver.

7. The solid state ultraviolet radiation detector of claim 3 whereinfirst and second metal contacts form an interdigited pattern on theas-grown surface.

8. The solid state ultraviolet radiation detector of claim 1 wherein thefirst barrier potential is greater than the second barrier potential.

9. The solid state ultraviolet radiation detector of claim 8 wherein thesecond metal contact is an ohmic contact.

10. The solid state ultraviolet radiation detector of claim 9 whereinthe second contact is an ultrasonically soldered indium rich alloy.

11. The solid state ultraviolet radiation detector of claim 10 whereinthe indium rich alloy comprises an alloy of 96.5% indium, 3% antimony,and 0.5% gallium.

12. The solid state ultraviolet radiation detector of claim 8 whereinthe first metal contact is a painted-on layer of silver.

13. The solid state ultraviolet radiation detector of claim 8 whereinthe first metal contact is an evaporated layer of silver.

References Cited UNITED STATES PATENTS 3,443,041 5/1969 Kahng et al.179-121 3,420,776 1/1969 Hepplewhite 252-69 3,560,812 2/1971 Hall317-234 OTHER REFERENCES Rideout et al.: Applied Physics Letters, vol.10, No. 11, June 1967, pp. 329-332 relied on.

JOHN W. HUCKERT, Primary Examiner M. H. EDLOW, Assistant Examiner U.S.Cl. X.R.

317-235 N, 235 UA, 237, 238; 250-833 UV, 211

